1. Field of the Invention
The invention relates to an organic electroluminescent display device, more particularly, to a front emitting and dual side emitting type organic electroluminescent display device having a low resistance cathode on a first transmissive type cathode to lower area resistance and to protect the first semi-transmissive type metal layer.
2. Description of Related Art
Generally, organic electroluminescent display devices adopt a rear emitting structure in which a transparent anode electrode, for example, an ITO layer, etc. is applied as a lower electrode, an organic film is formed as a multilayer structure, and a reflective cathode electrode, for example, Al, LiF/Al, LiF/Al, Li/Al, etc. is used as an upper electrode.
In the rear emitting structure, the cathode electrode of the device is an electrode for injecting electrons and it functions as a reflection film. The light from the organic electroluminescent emitting layer is transmitted towards the glass substrate (Applied Physics letter, Vol. 51,913 (1987)).
FIG. 1 is a cross sectional view of a conventional organic electroluminescent display device. The device comprises a glass substrate 10 and a transmissive type electrode or anode electrode 12, such as, an ITO layer or an electrode which functions similarly to the ITO layer. A thin film which functions as a hole injection layer 14 is formed on the anode electrode. A hole transport layer (not shown) may also be formed on the hole injection layer 14. Emitting layer 16 comprising a host doped with a dopant is formed on the hole transport layer. Cathode electrode 18 comprises of materials for easily injecting electrons, such as, Mg(Ag), LiF(Al), Li and Al.
The organic electroluminescent display device has a high luminance of about 100 to 10,000 cd/m2 at a low voltage of about 0 to 10 V.
Furthermore, a front emitting structure in which electrons are injected by minimizing the thickness of a semi-transmissive type cathode electrode to 10 nm or less, and emitted light is transmitted to both sides by a coating protection layer on the transparent electrode is known (SID 96 DIGEST. 185 42.2: Novel transparent organic electroluminescent devices, G, Gu, V. B. Bulovic, P. E. Burrows, S. R. Forrest, M. E. Tomson). The cathode electrode of the device adopts a structure in which an ITO layer is formed on an Mg—Ag layer having a thickness of 100 nm.
Furthermore, in U.S. Pat. No. 5,969,474, a structure in which a IZO (zinc doped indium oxide) layer is formed on a metal having a work function of 4 eV to obtain a low resistance cathode electrode is disclosed.
Further, a structure in which an organic electroluminescent display device is formed on an opaque substrate such as Si is disclosed in U.S. Pat. No. 5,714,838. This patent discloses the use of ITO and Al as a lower layer, and a transparent material such as ZnS, GaN, ITO and ZnSe for the cathode electrode.
A front-emitting structure in which a metal is used as a reflection film anode electrode, and a transmissive type electrode is used as the cathode electrode is known (SID 2001; 384 24.4L: A 13.0-inch display with top emitting structure and adaptive current mode programmed pixel circuit; Tatsuya sasaoka, et al).
However, the front-emitting structure has defects in area resistance since a metal is used as a single reflection film anode electrode, a semi-transmissive type metal of Mg(Ag) is thinly covered as a transmissive type cathode electrode, and a conductive oxide film is introduced onto the thin semi-transmission type metal of Mg(Ag). The conductive oxide film provides resistance and serves as a protection layer for the semi-transmissive film.
FIG. 2 is a cross sectional view illustrating structure of a conventional dual side emitting type organic electroluminescent display device.
Referring to FIG. 2, the conventional organic electroluminescent display device comprises a reflection film anode electrode 22, a layer 14 for injecting or transporting holes, an emitting layer 16, a semi-transmissive type conductive cathode electrode 18 and a transmissive type electrode layer 20.
Furthermore, cathode electrode technology for substantially increasing transmittance using organic materials without the use of a metal film for easily injecting electrons is patented for a front emitting or a dual side emitting structure as disclosed in U.S. Pat. No. 6,469,437.
However, although such a front emitting or dual side emitting structure functions to increase transmittance, the structure has defects in that driving voltage is increased since electron injection is radically deteriorated compared to a structure using metallic cathode electrodes.
The thickness of ITO is limited to about 1,200 Å due to cracks in a structure of Mg—Ag or ITO currently being studied as part of a transparent cathode electrode. The area resistance is measured to be about 35 Ω/m2 or less (Journal of Applied Physics Vol. 87 p 3080: Semitransparent cathode for organic light emitting devices). The power consumption tends to increase by driving voltage difference of about 2 V or more caused by area resistance in the resistance.
A ITO-Ag-ITO structure was introduced to construct a radio-wave absorbing panel in U.S. Pat. No. 6,195,034. Cases involving the study of the multilayer film including Ag as a transmissive type cathode electrode of an organic electroluminescent display device is not known, although a multilayer film including Ag has been studied in various other fields (Thin Solid Films 341 (1999) 152–155)(J. Szczyrbowski, A. Dietrich, K. Hartig, Solar Energy Mater. 19(1989) 43).
These conventional technologies have limits in lowering the area resistance of the transmissive type cathode electrode of a front emitting device. Therefore, it is very difficult to construct a large sized device. Since devices according to the conventional technologies tend to have the film peel off and to form cracks when the thickness of conductive transmission type oxide film is 150 nm or more, it is disadvantageous to form conductive oxide film of ITO using sputtering at an ordinary temperature. Furthermore, devices according to the conventional technologies are disadvantages because the film cannot be made thicker in order to lower area resistance accordingly.